http://code.google.com/p/openmetaverse/wiki/AddUserData says:

"Set a key to an empty string to remove the key/value pair"

but current code actually sets the value to an empty string instead of removing it. Is this a bug, or is the doc incorrect? I'm guessing it's a bug, but will confirm with jhurliman when I see him on IRC.

Bug. Best solution was to add RemoveUserData.